Application of uniform design in optimisation of three stage ageing of Al-Cu-Mg alloys

The hardness variation and the behaviour of precipitation during the process of three stage ageing of three Al-Cu-Mg alloys were investigated using micro-hardness and differential scanning calorimetry (DSC). To facilitate the determination of the optimised heat treatment condition, a uniform design method was employed by which the influences of three stages interrupted ageing on the hardness and precipitation can be elucidated with a limited number of experiments. It is found that optimised heat treatment with maximum hardness can be achieved for non-stretched materials by applying the method of three stage ageing, but not for stretched T351 materials. The selection of a long initial Stage I and a suitable Stage II ageing time are important, and the Stage II ageing at 25°C shows more beneficial effect than ageing at 65°C. The hardness during the three stage ageing is closely related to the amount of S phase formation.